In central air conditioning systems typically used in residences, a compressor is operable to circulate a vapor compression refrigerant between an indoor heat exchanger and an outdoor heat exchanger. In recent years, scroll-type compressors for the most part have replaced reciprocating-type compressors in residential air conditioning systems. For cost reasons, such scroll-type compressors typically do not include vibration-dampening springs to isolate the motor and compressor mechanism from the outer housing of the compressor. The refrigerant lines on the suction and discharge sides of the compressor are rigidly attached to this outer housing. Therefore, unlike older-style reciprocating compressors, there is a direct vibration transmission path to these refrigerant lines.
In particular the refrigerant line between the indoor heat exchanger and the compressor is susceptible to such vibrations because the line is relatively rigid due its relatively large diameter (e.g., ⅞ inch). In a non-heat pump air conditioning system, where the indoor heat exchanger operates as an evaporator, this refrigerant line corresponds to the compressor suction line, through which vapor refrigerant is drawn from the evaporator to the compressor. The length of such suction line may be about 40 feet, with most of the line being inside the building that is serviced by the air conditioning system. In a heat pump system, this refrigerant line corresponds to the compressor suction line when the system is operated in a cooling mode and to the compressor discharge line when the system is operated in a heating mode.
Such vibrations in the refrigerant line between the compressor and indoor heat exchanger may cause a droning noise that is readily detectable by occupants of the building. This droning noise results when a 120 Hz and/or 240 Hz vibration, which is typically associated with electric motor noise, is modulated by a low frequency (2 Hz or less) standing wave in the refrigerant line, which varies the intensity of the 120 Hz vibration and/or 240 Hz vibration. The standing wave causes displacement of the refrigerant line, such that contact between the line and a wall, floor or other structural component results in points of noise transmission inside the building.
One solution that has been proposed to inhibit such vibrations is to strap one or more strips of rubber around the refrigerant line, which reduces vibration by adding mass to the line and by frictional damping. The length of each rubber strip preferably corresponds to ¼ of the wavelength of the standing wave vibration (e.g., 24 inches). This solution typically is used as a “field fix” after the system installer has received a complaint about noise from a customer. The number of ¼ wavelength rubber strips needed is determined in the field, largely by trial and error. Further, the rubber strips are typically wrapped around sections of the refrigerant line that are external to a cabinet in which the outdoor heat exchanger and compressor are housed.